The present invention relates to a device for analyzing gaseous samples comprising a device for generating a plasma, a feeding device for the sample to be analyzed and at least one detector unit, which in particular comprises at least one interference filter, a lens arrangement and a photodetector for detecting radiation, especially atomic emission or molecular emission, emitted by the sample to be analyzed.
A device for analyzing gaseous samples of this kind has for example been disclosed in U.S. Pat. No. 5,009,099, wherein a plasma generator, to which the sample can be introduced, is followed by a detector unit consisting of a multitude of photodetectors wherein the object of this known device is in particular to provide background correction in a plasma gas chromatography detector.
A detector unit for use in a device, as mentioned above, to record the radiation emitted from a sample to be analyzed has in its own been disclosed for example in DE-OS 39 42 375, wherein it is the object of this known device to simultaneously record several wavelength ranges of a polychromatic radiation, with one detector for each of the wavelength ranges considered. In this known embodiment the detector or each detector unit is formed of an interference filter, a lens arrangement and a following photodetector with an according readout electronics.
A modified sensor for a portable analysis instrument can be found for example in DE-A 38 40 106, wherein a plurality of folding mirrors is employed.
For the production of a plasma for various applications it is furthermore referred to for example U.S. Pat. No. 4,654,504, U.S. Pat. No. 5,394,090, U.S. Pat. No. 5,394,091, U.S. Pat. No. 5,394,092 or U.S. Pat. No. 5,153,519, wherein in these known embodiments various designs of microwave and RF plasma generators are proposed to provide a plasma for various applications or analytical procedures. Problematic with these known devices is the coupling of electromagnetic energy into the plasma gas, wherein the employed power is usually around several hundred watts. Therefore the power to be coupled is very high, so that adequate heat dissipation has to be provided in immediate vicinity of the produced plasma, to avoid damage to parts of the apparatus. For this purpose not only parts of the apparatus must be made of an electrically non-conducting, high-temperature resistant material to separate the gas or plasma from the remaining parts of the apparatus, but furthermore by providing such enclosing elements for the plasma, there results mostly an enormous requirement of space for the corresponding cooling devices, which renders the production of a plasma of low spatial spread more difficult or even impossible.
In the context of producing the plasma and in particular the use of additive gases and the correspondingly used plasma gases it can furthermore be referred to U.S. Pat. No. 4,776,690, U.S. Pat. No. 5,151,371, U.S. Pat. No. 3,887,280 or U.S. Pat. No. 4,517,824, where the use of various plasma gases and their additive gases is described in the context of specific applications in the analysis of gaseous samples.
Regarding the selection of plasma gases the use of a noble gas, and in particular helium or argon, has been for example disclosed in DE-A 41 10 343 or U.S. Pat. No. 5,394,092.
Starting from a device of the kind mentioned above and taking into account the disadvantages of known devices and designs, it is the object of the present invention to provide a device for the analysis of gaseous samples by which it is possible, with a simple and compact design and low plasma power and a small detector volume, which favors use of the device according to the invention in mobile or portable instruments, to achieve a correspondingly high accuracy and reliability of results on the samples to be analyzed.
To solve this problem, the device of the subject invention is essentially characterized in that the device for generating the plasma is made up of two in particular ring- or disk-shaped parallel, interspaced electrodes, each having one essentially centrical, in particular circular through-opening, and an isolator having a particularly circular through-opening for confining the plasma and that between said device for generating the plasma and the detector unit an optical unit, in particular a collimator lens for generating a parallel ray beam is provided. By forming, according to the invention, the device for generating a plasma from at least one isolator, which is positioned between parallel interspaced, in particular ring- or disk-shaped electrodes, wherein the electrodes and the interposed isolator forming the plasma producing unit are each provided with an essentially centrical opening, the establishment a plasma of low spatial spread and defined position is successfully achieved, wherein the dimensions of the plasma can be selected according to the various requirements. Furthermore it is possible to achieve through said isolator, in the particularly circular through-opening of which said plasma is produced and maintained, in a simple way and without the provision of (additional) confining elements, such as tubes, or additional means for heat dissipation, a safe confinement of said plasma and simultaneously securing heat dissipation from the immediate vicinity of said plasma. Through said in particular ring- or disk-shaped electrodes, which are positioned at both sides of said isolator and the through-openings of which are aligned with respect to each other, the supply of the energy necessary for the ignition and maintenance of said plasma is successfully achieved in a very small volume, such that overall a simple method for the production of such a low-power plasma, in particular noble gas plasma, can be provided at low power uptake and low gas consumption. Apart from the device for generating a plasma being formed from simple elements and overall small dimensions, it is possible to provide, by combination with the further provided optical unit, in particular a collimator lens for the formation of a parallel ray beam, and at least one detector unit an altogether compact, easy to use and simple to maintain overall design of the design according to the invention.
According to a preferred embodiment of the device for analyzing gaseous samples according to the invention, the design is such that, viewed with respect to the direction of plasma gas flow, another isolator with a through-opening, which is essentially equivalent to said through-opening of said isolator positioned between said electrodes, is positioned upstream of said first electrode. By an isolator positioned upstream, with respect to the direction of supply of plasma gas, a shielding action towards the direction of supply of plasma gas and sample is achieved, such that an impairment of the plasma gas to be introduced, and the sample to be introduced and subsequently analyzed, can be prevented. At this in particular the provision of a suitably small through-opening in said upstream isolator, which is essentially equivalent to the through-opening of the isolator positioned between the electrodes, in which the plasma is formed and confined, it is possible to achieve protection of the sample against decomposition or polymerization, caused by UV radiation or extended glow discharges, before reaching the plasma.
The isolator positioned upstream of said plasma could, if this side is at ground potential, be made of metal, for example Pt/Ir. To reduce the number of components it is proposed in another preferred embodiment that the first electrode, viewed with respect to the direction of gas flow, and the isolator positioned upstream of it are combined into one single component, and that the through-opening corresponding to the through-opening confining the plasma is followed by a preferably conically expanding section.
To protect operative equipment which is positioned downstream of the system of said electrodes and said interposed isolator it is proposed to position an additional isolator downstream of the, viewed with respect to the direction of gas flow, second electrode, the through-opening of said isolator being preferably slightly smaller than the through-opening of the adjacent electrode, which constitutes another preferred embodiment of the device according to the invention. By selecting the through-opening of the downstream isolator appropriately, protection of the electrode surface is improved. Furthermore the fact that the through-opening of the electrodes is considerably larger than the inner diameter of the through-opening of at least the isolator confining and defining the plasma, ensures that a sufficient amount of the radiation emitted by the sample to be analyzed, in particular atomic or molecular emission, can escape from the plasma production unit for subsequent detection. In this context it is proposed according to a particularly preferred embodiment that the internal diameter of the through-opening in the isolator positioned downstream of said electrodes is at least two times the internal diameter of the through-opening in the isolator positioned upstream of said electrodes.
To simplify the assembly of the device to generate the plasma, such that the central part for plasma production being constituted by the electrodes and the isolator can be for example pre-manufactured, it is proposed according to another preferred embodiment that said electrodes and isolators are either pressed together mechanically, for example by spring action, or are bonded together by known techniques of metal-ceramic bonding, in particular by soldering in vacuum or hydrogen atmosphere.
For a particularly simple mounting of the plasma production unit it is further preferably proposed that said electrodes and said isolator or isolators are held in at least one fixture and are mounted in a gas-tight manner, wherein in particular due to the fact that the spatial dimensions of the plasma are extremely small, it is further preferably proposed that the fixtures are equipped with centering mounts for said electrodes and/or isolators. In such a way a pre-assembled unit or device for producing a plasma can be easily replaced as a whole, wherein the centering mounts of the fixtures enable precise and reliable positioning relative to the remaining elements, and in particular relative to the inlet of the plasma gas and the sample to be analyzed as well as the downstream optical unit and detector unit.
As already mentioned above, the implementation of a compact device for generating a plasma, which can be operated at a suitably low power level, renders the provision of costly accessory equipment unnecessary, wherein in particular the provision of additional, often bulky cooling devices can be eliminated. For an orderly exhaust of the plasma gas and/or the supply of an additive gas in the region of plasma production it is, according to the invention, furthermore preferably proposed that said fixtures have outlets or purging holes, in particular for supplying an additive gas to the plasma gas. Regarding the exhaust of the plasma gas it can be further provided that the cavity housing said electrodes and/or said fixtures can be purged with a purge gas, according to a further preferred embodiment of the device according to the invention. In particular the provision of a purge, for the cavity housing the electrodes and/or said fixtures, with a purge gas produces benefits with respect to requirements of gas-tightness for the electrode assembly, as in this case the specifications for gas-tightness for the device for producing the plasma can be lowered. Furthermore, by such a purge gas the plasma exhaust gases can be exhausted in a controlled way and, if needed, consequently be filtered, wherein for the exhaust of plasma gases by such a purge gas attention must be paid to sufficiently dilute the plasma gas with a plasma-impeding gas to prevent uncontrolled discharges in the region outside the device for plasma production, and in particular in the region of energy supply. Similarly it may be provided that the space between said device for generating the plasma and said optical unit can be purged with a purge gas, according to another preferred embodiment of the device according to the invention.
As already mentioned above, the device for generating a plasma with narrow spatial dimensions, provided in the device according to the invention, enables the production of an accordingly low-power plasma, wherein according to the invention it is provided in this context that the power of the plasma is below 50 W, and preferably between 3 and 30 W, such that sufficient heat dissipation can be accomplished without the provision of costly cooling devices. For an array of plasma discharges said power is provided for each single discharge.
In order to accomplish a stable plasma with simple and cost-effective electronic components the device according to the invention is preferably characterized in that the excitation or operating frequency for said device for generating the plasma is selected to be at least 5 kHz, preferably in the range of 50 kHz to 5 GHz, and more preferably above 10 MHz.
As to the plasma gas it is preferably proposed for applying the device according to the invention that the plasma gas is selected from helium or argon. In particular helium is preferred because of its low atomic mass, since it hardly causes erosions on the electrodes.
According to a further preferred embodiment it is proposed that the pressure of the plasma gas is selected to be at least 0.01 bars, preferably between 0.1 and 5 bars, so that particularly around atmospheric pressure a low-power plasma for the analysis of samples can be provided.
For the formation of a plasma it may be provided besides the use of plasma gas in various applications that an additive gas is admixed to said plasma gas at a level of at most 35 vol.-%, preferably less than 25 vol.-%, wherein said additive gas is preferably selected from CO2, N2, air, water vapor, hydrogen and oxygen, according to another preferred embodiment. Furthermore a vaporized compound, in particular water vapor, may be provided by diffusion or permeation in a thermostatted device close to the plasma, whereby the generally difficult transport of the vapor to the plasma through for example heated ducts can be avoided.
For the simultaneous evaluation or analysis of different wavelength ranges it is proposed according to a further preferred embodiment that several detector units are arranged side by side and are illuminated by the parallel ray beam, wherein it is provided according to a particularly preferred embodiment that a multitude of detector units are positioned each at the same distance from and around a centrally positioned detector unit. By providing a multitude of detector units side by side in the emitted ray bundle it is possible to obtain simultaneous, fast and reliable analysis results. Herewith the interference filters assigned to a single detector unit can be used in a mounting which ensures the best possible resolution or transmission curve, wherein the arrangement of multiple detector units side by side furthermore enables a very simple assembly and if needed a simple interchange of single detector units or element combinations, which each consist of a filter, a lens assembly and a photodetector with possibly integrated signal electronics. The use of only a single detector unit can be considered for the analysis of elements such as carbon, hydrogen, oxygen or nitrogen, for which a spectral background correction is not required, since essentially the baseline is dominated by a measurable signal, caused by contamination. The arrangement of multiple detector units side by side can be used for the analysis of a corresponding number of different elements, or for example also to analyze multiple different lines of a single element, to increase selectivity, wherein generally wavelength combinations covering a very wide range, as well as for special applications having arbitrarily close spacing, are possible. For the determination of a correction signal it may be provided according to a particularly preferred embodiment that the centrally positioned detector unit has an area smaller than that of the other detector units, wherein in such a centrally positioned detector unit of reduced area for example oxygen can be measured for correction purposes.
For an accordingly efficient use of the emitted radiation, which can be very positively affected by suitable selection of the through-opening of the downstream isolator in the device for plasma generation, it is proposed according to a further preferred embodiment that collimator lens is formed by an aspheric collimator lens of high aperture.
For special applications a separation of the device for plasma generation from the actual analysis or detector device may be advantageous, wherein it is proposed that between said device for generating the plasma and the at least one detector unit a fiber-optic link is provided, according to another preferred embodiment of the device for analyzing gaseous samples according to the invention.
For a further optimization of space requirement it may be provided according to a further preferred embodiment, that between said device for generating the plasma and the at least one detector unit a deflecting or folding mirror is provided.
When using the device for analysing gaseous samples according to the invention it is further possible to use a carbon signal to protect the plasma from overload, wherein in the case of overload of the plasma with organic substances soot may be formed in the discharge zone and, due to the electrical conductivity of the soot, may create problems. Such the continuous measurement of an appropriately scaled carbon signal from the plasma may be used to switch off the plasma for a predetermined time when a certain threshold is exceeded, whereupon the plasma may be re-ignited automatically. Such an automatic control, which may also take into consideration the ratio of carbon to oxygen, can be implemented easily by providing the appropriate electric or electronic circuitry. In this context it is preferably proposed that said device for generating the plasma is coupled to an automatic control for switching off said device for generating the plasma as soon as a threshold value for the carbon signal is exceeded, and subsequent re-ignition of the same.
Further, by providing a multiple detector units positioned side by side a relatively simple background correction can be implemented, wherein again the large solid angle, which can be obtained by suitable dimensioning of the exit aperture of the device for plasma generation, enables the simultaneous measurement of a multitude of wavelength ranges and the resulting assistance in background correction.
Signal processing of the various signal and background intensities obtained by separate measurement with photodetectors can further be achieved with the help of at least partially known evaluation circuitry or microcomputers.